1. Field of the Invention
The present invention relates to a laser diode and a method for fabricating the same, and more particularly, to a laser diode that uses air as a reflective layer, thereby enhancing reflectance with respect to an oscillating laser beam, and a method for fabricating such a laser diode.
2. Description of the Related Art
In general, a semiconductor laser diode is a structure in which a buffer layer, a laser oscillating layer that is composed of a buffer layer, a lower clad layer, an active layer and an upper clad layer, and a cap layer and an upper electrode are sequentially stacked. Also, a lower electrode is formed below the semiconductor substrate.
When a predetermined voltage is applied across the upper electrode and the lower electrode, an electromagnetic wave of predetermined wavelength, which is generated in the active layer and oscillates between the both sides of the laser oscillating layer, is emitted out of a side of the laser oscillating layer. This is an edge emitting laser diode. Laser diodes have various uses, but have been mainly used in optical pickups for writing and reading data on a writing medium such as digital versatile discs (DVDs).
It is required that a mirror layer with a high reflectance be attached to a side of the laser oscillating layer so as to generate a high-output laser beam. The mirror layer is obtained by coating a thin layer that has a high reflectance on one side of the laser oscillating layer. Today, there is a lot of ongoing research into the formation of such a thin layer.
In the past, as can be seen in FIG. 1, a reflective layer 15 was obtained by alternatively depositing first and second dielectric layers 13 and 14 which have different refractive indexes several times on a side of a laser oscillating layer 12 from which laser light is emitted, thereby making a laser diode whose threshold voltage is reduced and which can produce a high-output laser beam. Here, the thickness of each of the first and second dielectric layers 13 and 14 is calculated by xcex/(4n), wherein xcex denotes the wavelength of laser beam emitted from the laser oscillating layer 12, and n denotes the refractive index of each dielectric layer with respect to the wavelength of laser beam emitted. In general, the greater the difference between the refractive indexes of two dielectric layers, and the greater the number of layers deposited are, the greater the reflectance is. Here, the dielectric layers 13 and 14 are alternately deposited several times to form several pairs of the dielectric layers 13 and 14.
It is possible to select various dielectric materials for the dielectric layers 13 and 14 which make up the reflective layer 15, according to the wavelength of laser beam emitted from the laser oscillating layer 12. Preferably, the dielectric layers are formed of SiO2 and TiO2 because the difference between their refractive indexes is greater than any two other dielectric materials. Once dielectric materials are selected, the first dielectric layer 13 is formed, and then second dielectric layer 14 whose refractive index is larger than that of the first dielectric layer 13 is formed. If these dielectric layers 13 and 14 are formed of SiO2 and TiO2, the difference between their refractive indexes is about 1.35 with respect to a laser wavelength of 410 nm. To give the reflective layer 15 a high reflectance, e.g., 90% or more, it must be made of at least three pairs of the first and second dielectric layers 13 and 14. Therefore, for high reflectance, the number of pairs of the dielectric layers 13 and 14 must be greater than a predetermined number. However, as the number of pairs of the dielectric layers 13 and 14 increases, it takes more time to form and etch them to make a thin layer.
Hereinafter, a conventional reflective layer and its manufacturing process will be described with reference to FIGS. 2A through 2C. Referring to FIG. 2A, a laser oscillating layer 22 from which a laser beam is emitted is deposited on a semiconductor substrate 21, and then, two dielectric materials are alternatively deposited to form dielectric layers 23 and 24 that together make up a reflective layer 25. When the dielectric layers 23 and 24 are formed, a dielectric material whose refractive index is comparatively low is first deposited, and then a dielectric material whose refractive index is relatively high is later deposited. The dielectric layers 23 and 24 are alternately deposited several times to form a desired number of pairs thereof.
As described above, in order to obtain a reflectance of 90%, at least three pairs of the dielectric layers 23 and 24 must be deposited, even if they are formed of SiO2 and TiO2 which have a greater difference in refractive index than any two other materials, as shown in FIG. 2B. During this process, several pairs of the dielectric layers 23 and 24 are deposited on a portion of the semiconductor substrate 21 as well as a side of the laser oscillating layer 22 from which a laser beam is emitted. Thereafter, a portion of the several pairs of the dielectric layers 23 and 24 formed on an upper electrode 26 must be removed to connect the upper electrode 26 with the outer electrode as shown in FIG. 2C. This is because the dielectric layers 23 and 24 are non-conductive layers that block the flow of current. Accordingly, a conventional laser diode is disadvantageous in that it takes a lot of time to deposit and etch the dielectric layers 23 and 24, especially, when there are many pairs of the dielectric layers 23 and 24.
To solve the above problem, it is a first object of the present invention to provide a laser diode in which the number of pairs of dielectric layers is reduced due to an air layer formed on a dielectric layer with a low refractive index, and a reflective layer of higher reflectance is formed, thereby giving the laser diode a reduced threshold voltage and a high output.
It is a second object of the present invention to provide a method for fabricating such a laser diode.
To achieve the first object, there is provided a laser diode including a substrate, a laser oscillating layer formed on the substrate, an upper electrode formed on the laser oscillating layer, and a reflective layer formed at one side of the laser oscillating layer, wherein the reflective layer comprises air layers.
Preferably, the reflective layer has a structure in which an air layer and a dielectric layer are alternately deposited several times and is made of at least two pairs of an air layer and a dielectric layer.
Preferably, the dielectric layers include a TiO2 layer.
To achieve the second object, there is provided a method of fabricating a laser diode including the steps of: (a) forming a laser oscillating layer on a substrate, and then, forming an upper electrode on the laser oscillating layer; (b) forming sacrificial layers and dielectric layers alternately at one side of the laser oscillating layer; and (c) etching the sacrificial layers selectively.
Preferably, during step (b) the sacrificial layers and the dielectric layers are alternately deposited to form two pairs of these layers.
Preferably, during step (b), the sacrificial layers and the dielectric layers are formed by sputtering.
Preferably, this method further includes removing the sacrificial layers and the dielectric layers from the upper electrode after the step (b), and the sacrificial layers and the dielectric layers are removed from the upper electrode by RIE.
Preferably, during step (c), the sacrificial layers are selectively etched by wet etching, using a buffered oxide etchant (BOE).